Conventionally, in an injection molding machine, resin heated and melted in a heating cylinder is injected into the cavity of a mold apparatus under high pressure so that the cavity is filled with the resin. The molten resin is then cooled and solidified so as to obtain a molded article.
For performing such molding operation, the injection molding machine includes a mold clamping apparatus and an injection apparatus. The mold clamping apparatus is provided with a stationary platen and a movable platen. The movable platen is advanced and retracted by a mold clamping cylinder, to thereby perform mold closing, mold clamping, and mold opening.
The injection apparatus includes a heating cylinder for heating and melting resin supplied from a hopper, and an injection nozzle for injecting the molten resin. Further, a screw is disposed within the heating cylinder such that the screw can be rotated and can be advanced and retracted. The screw is advanced so as to inject the resin from the injection nozzle, and is retracted so as to meter the resin.
In order to advance and retract the screw, a motor-driven injection apparatus is provided.
FIG. 1 is a schematic view of a conventional injection apparatus.
In FIG. 1, reference numeral 2 denotes an injection apparatus, and 4 denotes a frame of the injection apparatus 2. A heating cylinder 21 is fixedly provided in front (left side in FIG. 1) of the frame 4, and an injection nozzle 21a is provided at the front end (left-side end in FIG. 1) of the heating cylinder 21. A hopper 21b is disposed on the heating cylinder 21, and a screw 20 is disposed within the heating cylinder 21 such that the screw 20 can be rotated and can be advanced and retracted (moved leftward and rightward in FIG. 1, respectively). The rear end (right-side end in FIG. 1) of the screw 20 is rotatably supported by a support member 5.
Attached to the support member 5 is a metering motor 6 having a speed reduction mechanism. The rotation of the metering motor 6 is transmitted to the screw 20 via a timing belt 7a. 
Further, a screw shaft B is rotatably supported in parallel with the screw 20. The rear end of the screw shaft 8 is connected, via a timing belt 7b, to an injection motor 9 having a speed reduction mechanism. That is, the injection motor 9 is adapted to rotate the screw shaft 8. The front end of the screw shaft 8 is in screw engagement with a nut 5a fixed to the support member 5. Accordingly, when the injection motor 9 is driven, the nut 5a can be moved axially through rotation of the screw shaft 8 via the timing belt 7b. 
In the injection apparatus 2 having the above-described structure, during a metering stage, the metering motor 6 is driven in order to rotate the screw 20 via the timing belt 7a, thereby retracting the screw 20 by a predetermined distance (rightward in FIG. 1). At this time, resin is supplied from the hopper 21b, heated and melted within the heating cylinder 21, and accumulated forward of the screw 20 as the screw 20 retracts.
Further, during an injection stage, the injection motor 9 is driven in order to rotate the screw shaft 8 via the timing belt 7b, so that the nut 5a and the support member 5 are moved with the rotation of the screw shaft 8. As a result, the screw 20 is advanced (moved leftward in FIG. 1), and the resin accumulated forward of the screw 20 is injected from the injection nozzle 21a. 
However, the injection apparatus 2 has the following drawbacks. That is, in the injection apparatus 2, the metering motor 6 and the injection motor 9 must be driven during the metering stage and the injection stage, respectively. Further, rotation of the metering motor 6 and rotation of the injection motor 9 are transmitted to the screw 20 via speed reduction mechanisms, pulleys, etc. Therefore, mechanical efficiency is comparatively low, and inertia is comparatively high. As a result, during the injection stage, reaching an initial injection speed and changing the injection speed require a comparatively long period of time and a comparatively large torque. Further, the time required to proceed from the injection stage to the pressure holding stage is comparatively long.
In order to overcome the above-described drawbacks, there has been provided a built-in-motor-type injection apparatus in which a screw, an injection motor, and a metering motor are disposed on a common axis.
FIG. 2 is a sectional view of such a conventional built-in-motor-type injection apparatus.
In FIG. 2, reference numeral 12 denotes a heating cylinder, and an injection nozzle 12a is provided at the front end (left-side end in FIG. 2) of the heating cylinder 12. A screw 22 is disposed within the heating cylinder 12 such that the screw 22 can be rotated and can be advanced and retracted (moved leftward and rightward in FIG. 2).
The screw 22 has a screw head 22a at its front end. The screw 22 extends rearward (rightward in FIG. 2) within the heating cylinder 12, and is connected at its rear end (right-side end in FIG. 2) to a first spline shaft 63.
Therefore, during a metering stage, when the screw 22 is retracted (moved rightward in FIG. 2) by a determined distance, while being rotated, resin in the form of pellets is supplied from an unillustrated hopper, heated and melted within the heating cylinder 12, and accumulated forward (leftward in FIG. 2) of the screw head 22a as the screw 22 retracts.
Further, during an injection stage, when the screw 22 is advanced (moved leftward in FIG. 2), the resin accumulated forward of the screw head 22a is injected from the injection nozzle 21a and charged into a cavity of an unillustrated mold apparatus.
A drive section casing 11 is fixed to the rear end of the heating cylinder 12. A metering motor 44 is disposed at the front portion (left-side portion) of the drive section casing 11 and an injection motor 45 is disposed at the rear portion (right-side portion) of the drive section casing 11 such that the metering motor 44 and the injection motor 45 share a common center axis. The metering motor 44 comprises a stator 46 and a rotor 47, and the injection motor 45 comprises a stator 48 and a rotor 49.
The rotor 47 is supported to be rotatable relative to the drive section casing 11. Specifically, a hollow first rotor shaft 56 is fixedly fitted into the rotor 47, and the first rotor shaft 56 is supported by bearings 51 and 52.
Similarly, the rotor 49 is supported to be rotatable relative to the drive section casing 11. Specifically, a hollow second rotor shaft 57 is fixedly fitted into the rotor 49, and the second rotor shaft 57 is supported by bearings 53 and 54.
The screw 22 can be retracted, while being rotated, through drive of the metering motor 44. In order to enable this movement, a first spline nut 62 is fixed to the front end of the first rotor shaft 56; a first spline shaft 63 is in spline-engagement with the first spline nut 62; and the screw 22 is fixed to the front end of the first spline shaft 63. Therefore, when the rotor 47 is rotated through drive of the metering motor 44, rotation of the rotor 47 is transmitted to the screw 22, so that the screw 22 rotates. At this time, the first spline shaft 63 is retracted relative to the first spline nut 62, so that the screw 22 is retracted. It is to be noted that when the screw 22 is retracted, back pressure is applied to the screw 22 against pressure generated by the resin.
Further, the screw 22 can be advanced through drive of the injection motor 45. In order to enable this movement, an annular bearing retainer 64 is fixed to the rear end of the second rotor shaft 57; and a ball screw shaft 65 is inserted into and fixed to the bearing retainer 64. The ball screw shaft 65 is supported to be rotatable relative to the drive section casing 11. Specifically, the ball screw shaft 65 is supported on the drive section casing 11 via the bearing retainer 64 and a bearing 66, as well as via a bearing 67 disposed on the rear side of the bearing 66.
A ball nut 69 is disposed within the second rotor shaft 57 such that the ball nut 69 can advance and retracts, and is in meshing-engagement with the ball screw shaft 65. Accordingly, rotation of the rotor 49 is transmitted to the ball screw shaft 65 via the second rotor shaft 57 and the bearing retainer 64. The ball nut 69 and the ball screw shaft 65 convert rotational motion to linear motion, so that the ball nut 69 is advanced and retracted.
Further, in order to prevent the ball nut 69 from rotating together with the ball screw shaft 65, a hollow second spline shaft 71 is fixed to the front end of the ball nut 69, and the second spline shaft 71 is in spline-engagement with a second spline nut 76 fixed to the drive section casing 11.
A bearing box 72 is fixed to the front end of the second spline shaft 71. A thrust bearing 73 is disposed within the bearing box 72 to be located at the front side thereof, and a bearing 74 is disposed within the bearing box 72 to be located at the rear side thereof. Accordingly, the first spline shaft 63 is supported by the bearings 73 and 74 to be rotatable relative to the second spline shaft 71 and the ball nut 69.
In the above-described structure, rotation of the metering motor 44 and rotation of the injection motor 45 are transmitted to the screw 22 without intervention of a speed reduction mechanism, pulleys, etc. Therefore, mechanical efficiency increases, and inertia decreases.
The drive section casing 11 is formed of a front cover 11a, a center casing 11b, and a rear cover 11c; and the heating cylinder 12 is fixed to the front end of the front cover 11a. 
The metering motor 44 is surrounded by a sleeve-shaped stator frame 46a, and the injection motor 45 is surrounded by a sleeve-shaped stator frame 48a. The front cover 11a and the center casing 11b are connected together by use of threaded rods 46b, with the stator frame 46a being sandwiched between the front cover 11a and the center casing 11b. Similarly, the center casing 11b and the rear cover 11c are connected together by use of threaded rods 48b, with the stator frame 48a being sandwiched between the center casing 11b and the rear cover 11c. The stator frame 48a is supported by means of frictional force generated through tightening of the rods 48b. 
In the above-described conventional injection apparatus, since the metering motor 44 and the injection motor 45 are disposed on the same axis, the injection molding machine is increased in axial length. When an attempt is made to reduce the injection molding machine in axial length, the outer diameters of the metering motor 44 and the injection motor 45 increase, resulting in increased inertia.
Further, when the injection motor 45 is driven in order to rotate the ball screw shaft 65 to thereby inject the resin from the heating cylinder 12 through advancement of the screw 22, a reaction force corresponding to the injection force is transmitted to the rods 46b via the heating cylinder 12 and the front cover 11a, and to the rods 48b via the rear cover 11c. Therefore, the rods 46b and 48b extend, resulting in weakened tightening force.
In the above-described structure, when the rotor 47 or 49 is rotated upon drive of the metering motor 44 or the injection motor 45, the stator frame 46a or 48a may rotate. Therefore, the tightening force of the rods 46a and 48b must be controlled strictly. This makes assembly and maintenance of the injection molding machine more troublesome.
An object of the present invention is to solve the above-mentioned problems in the conventional injection apparatus, and to provide an injection molding machine which has improved mechanical efficiency and reduced inertia, which has a shortened axial length, and which facilitates assembly and maintenance.